The present invention relates to a microwave filter using a plurality of dielectric resonators, and more particularly to a microwave filter in which a plurality of resonators are formed in a single dielectric body.
A high frequency signal filter for the microwave band has been proposed which uses a dielectric resonator using dielectric material having a high permittivity. As is well known, a dielectric resonator has the following advantages.
(1) A dielectric resonator is small in size; a dielectric resonator having a size similar to a strip line resonator can be obtained.
(2) A dielectric resonator having an un-loaded Q whose value is approximately equal to that of a waveguide resonator can be obtained.
(3) The producing process of a dielectric resonator is simple as compared with that of a waveguide resonator.
On the contrary, a dielectric resonator has the disadvantage that is is difficult to obtain temperature characteristics required. However, recent progress of dielectric material engineering is solving this problem. Therefore, a filter having dielectric resonators will be increasingly utilized.
FIG. 1 is a plan view of a prior microwave filter using a plurality of dielectric resonators. On the opposite ends of a conductive rectangular case 19, there are provided an input terminal 11 to which an antenna 13 is connected and an output terminal 12 to which an antenna 14 is connected. Dielectric resonators 15-18 are spatially aligned between the antennas 13 and 14. Each of the resonators 15-18 is composed of a hollow cylindrical member made of dielectric material and a conductive rod which is inserted in the hollow member. One ends of the dielectric resonators thus configurated are secured to the bottom surface of the case 19. In order to obtain the desired filter response characteristics by using those dielectric resonators, the length (d) between the antennas 13, 14 and the dielectric resonators 15, 18, respectively is determined in accordance with the desired amount of the coupling therebetween. Likewise, the length between adjacent dielectric resonators is determined in accordance with the desired amount of the coupling therebetween. In this case, it will be understood that the amount of the coupling increases when the distance decreases, and the amount of the coupling decreases when the distance increases.
However, the prior microwave filter of FIG. 1 has the following disadvantages.
(1) Upon the assembling process of the filter, it is difficult to locate exactly the dielectric resonators on the bottom surface of the case in accordance with the design specification. A slight error in positioning the dielectric resonators brings about significant errors in the filter characteristics.
(2) In practice, it is difficult to secure the dielectric resonators on the bottom surface of the case.
(3) The production cost is high since the filter uses a plurality of individual dielectric resonators.
In order to overcome these disadvantages, the applicant has proposed another dielectric filter (see U.S. Pat. No. 4,386,328). This filter uses a plurality of dielectric resonators formed in a single dielectric block. FIG. 2 is a cross sectional view of a part of the filter shown in the above U.S. patent. In this figure, the reference numeral 20 is a rectangular block made of a dielectric material. The block 20 has through holes 21, 22 which are spatially disposed at a predetermined pitch (P) from each other. Each of the holes is covered with a conductive material. Then, each of the plated holes surrounded by the dielectric material acts as a dielectric resonator. Furthermore, the block 20 has a groove 23 disposed between adjacent plated holes 21 and 22. The groove 23 acts to transfer the electromagnetic wave resulting from the plated hole 21 to the plated hole 22, or vice versa. In other words, the groove 23 acts as an electromagnetic coupler between adjacent dielectric resonators. The amount of the coupling depends upon the size of the groove.
However, this type of the filter has the disadvantage that the producton cost is high and the production process is complicated, because the particular process for producing the groove in the dielectric block is required.